1. Field of the Invention
The present invention relates to a flow quantity measuring device, which measures a flow quantity of air.
2. Description of Related Art
For instance, a thermal-type flow quantity measuring device has been proposed to measure a flow quantity of air at an airflow passage, such as a flow quantity of intake air for an internal combustion engine (see for example, Japanese Unexamined Patent Publication No. 2000-193505 corresponding to U.S. Pat. No. 6,862,930). FIG. 5 illustrates one previously proposed flow quantity measuring device 100, which includes a dielectric film 101, a heating resistor 103, an upstream side sensing resistor 104 and a downstream side sensing resistor 105. The dielectric film 101 extends along a plane that is generally parallel to a flow of the air in an airflow passage. The heating resistor 103 is placed over a surface 102 of the dielectric film 101 and generates a heat upon energization. The upstream side sensing resistor 104 and the downstream side sensing resistor 105 are provided on an upstream side and a downstream side, respectively, of the heating resistor 103 on the surface 102. The electric resistances of the upstream side sensing resistor 104 and the downstream side sensing resistor 105 change in response to the temperature. A flow quantity of the air, which passes through the airflow passage, is measured based on a measurement difference between a measurement of the upstream side sensing resistor 104 and a measurement of the downstream side sensing resistor 105.
That is, in the flow quantity measuring device 100, an area, which is held between the upstream side sensing resistor 104 and the downstream side sensing resistor 105 on the surface 102 of the dielectric film 101, forms a sensing area 108, which senses the measurement difference. A heat generating area 109, which is formed by the heating resistor 103, heats the sensing area 108, so that the temperature distribution is created in a flow direction of the air in the sensing area 108. The measurement difference, which corresponds to the temperature distribution, is sensed and is used to determine the flow quantity of the air. A portion of the total amount of heat, which is applied from the heat generating area 109 to the sensing area 108, is used to heat the sensing area 108, i.e., is used to create the temperature distribution. It is conceivable that when a percentage of this portion relative to the total amount of heat is increased, the flow quantity measurement sensitivity is increased to reduce the measurement error.
The heat, which is applied from the heat generating area 109 to the sensing area 108, is not entirely used to heat the sensing area 108. That is, a portion of the heat, which is applied from the heat generating area 109 to the sensing area 108, is released without heating the sensing area 108. Particularly, terminals 110, 111 of the heating resistor 103 are directly connected to the heating resistor 103 to receive the heat from the heating resistor 103. Also, the terminals 110, 111 have a relatively low heat transfer resistance. Thus, the terminals 110, 111 serve as heat sinks for releasing the heat from the heat generating area 109.
In view of the above point, in the flow quantity measuring device 100, electric potential ends 114, 115 of the heating resistor 103, which are connected to the terminals 110, 111, are placed at a first end edge 116 of the sensing area 108, so that the releasing of the heat from the heat generating area 109 to the terminals 110, 111 is performed at the first end side of the sensing area 108. In this way, the amount of heat release to the terminals 110, 111 can be reduced in comparison to the case where the electric potential terminals 114, 115 are placed at the first end edge 116 and a second end edge 1171 respectively, of the sensing area 108 to perform the releasing of the heat from the heat generating area 109 to the terminals 110, 111 at the first end side and the second end side of the sensing area 108.
However, even when the electric potential ends 114, 115 are placed at the first end edge 116, the amount of heat released from the electric potential ends 114, 115 to the terminals 110, 111 is still relatively large. Therefore, it is demanded to further improve the flow quantity measurement sensitivity.